A controlled, replicated before-and-after experiment in January-April and October 1998, and March 1999, in Argyll, Scotland (Ross et al. 2001), found that an underwater playback system (UPS) was effective in deterring common eider Somateria mollissima from feeding on mussels Mytilus edulis at farms on two sea lochs (47-80% fewer birds feeding after use of the UPS; 2-37 birds feeding before use). Underwater recordings of an approaching ‘scare boat’ (scaring by boat being a conventional deterrent method) were played via an underwater loudspeaker, and also a ‘control’ i.e. playback of an unassociated sound. Average return time of eiders after chasing by boat also increased significantly, suggesting that effectiveness was strengthened by UPS.